The complexity of your situation is similar to multiple acquisitions I have encountered on both sides of the table in private and public transactions. In most situations involving private equity, it is doubtful if the additional cost of a private agent would generate a material gain given your minority position. However, since your situation has the additional complexities of differentiation in EBITDA contribution and negotiation with family members, I would agree with your recommendation to use an external agent. If the cost is a concern, you may want to consider using an advisor/coach/consultant to work on a fixed fee basis instead. I took this approach in selling one of the private companies I ran and was able to complete the transaction at a much lower cost than using an investment banker or agent. I'm happy to share my experience and lessons learned if you would like to follow up with a discussion.